A relatively simple procedure was developed for the preparation of tyramine-SSS-III, which could be radio-labeled with 125I to yield a product of high specific activity (2,000 cpm/ng), with no change in immunological or physical properties. This material was used successfully (a) in a radio-immunoassay (Farr test) to obtain precise and detailed information on the amount of serum antibody produced at intervals following immunization with different amounts of SSS-III, and (b) to examine the relationship between the distribution of SSS-III in the serum and tissue of immunized mice, and the kinetics for the appearance of serum antibody and antibody-forming cells. Almost all of the injected SSS-III was excreted in the urine during the first 48 hrs after immunization; some degradation took place in vivo. No evidence was found to support the view that the continuous recirculation of antigen influences the serum antibody levels obtained in mice given an optimally immunogenic dose of SSS-III, or plays a role in low-dose tolerance to this antigen.